Aluminum Tube. 19 mm, 20 mm, 21 mm. Length Options Actual product color may vary slightly due to different monitor settings. 21×20, 21×19, 21×18, 21×17, 21×16, 21×15, 21×14, 21×13, 21×12, 21×11, 21×10, 21×9, 21×8.